In the field of liquid crystal displays or electroluminescent (EL) displays, transparent conductive thin films typically formed of indium tin oxide (ITO) are patterned in certain shapes, and widely used as pixel electrodes and wires (which will be generally called "conductors"). Such a transparent conductive thin film, however, has a relatively high resistivity, which undesirably results in increased resistance in the wiring system. In order to reduce the resistance in the wiring using the transparent conductive thin film, it has been proposed to employ a laminated structure consisting of a transparent conductive thin film and a metal thin film (as disclosed in Japanese laid-open Patent Publications No. 5-307997 and No. 6-7180) in display devices that tend to be large-sized in recent years.
One type of the laminated structure is formed by laminating a metal semi-transparent film having a thickness of 20 nm or smaller and a transparent conductive thin film over the entire area of an electrode surface. This laminated structure, however, cannot be practically used since it has an insufficient light transmittance in its portion where pixels are present, and the resistance in the wiring system as a whole cannot be sufficiently reduced.
To solve the above problem, a laminate type conductor is employed wherein a metal thin film having a thickness of about 50 nm or larger is superposed on a portion of a transparent conductive thin film other than its portion that forms a matrix of pixels. The metal thin film is often formed of Al or an alloy containing Al as a major component, so as to achieve sufficiently reduced resistance. Such a laminate type conductor may be classified into two types which are different in the laminated structure of the transparent conductive thin film and metal thin film (or different in the order in which the films are formed). One of the two types is produced by forming the transparent conductive thin film on a substrate, patterning the thin film, and then forming and patterning the metal thin film. The other type is produced by forming and patterning the metal thin film on the substrate, and then forming and patterning the transparent conductive thin film.
The former type of laminated structure is disadvantageous in that the choice of a metal for forming the metal thin film is limited since an etching liquid used when patterning the metal thin film should not dissolve the transparent conductive thin film. In the case where the metal thin film is patterned by dry etching, too, the surface of the transparent conductive thin film may be etched and thus damaged.
The latter type of laminated structure is illustrated in FIG. 9. FIG. 9 is a schematic cross-sectional view showing a known example of thin-film laminate type conductor. A metal thin film 2 made of Al and a crystalline, transparent conductive thin film 3 made of ITO are formed in respective patterns on a substrate 1, such that a part of the conductive thin film 3 is laminated on a part of the metal thin film 2. This structure is disadvantageous in that projections called "hillocks" are formed on the metal thin film under the ITO thin film when the substrate is heated to 150.degree. C.-350.degree. C. during formation and heat treatment of the ITO thin film. The hillocks are reflected by the ITO thin film laminated on the metal thin film, and even larger hillocks are formed on the surface of the ITO thin film. The hillocks thus formed may cause abnormalities, such as short-circuiting with opposed electrodes, which considerably deteriorate the reliability of the display device.
FIG. 10 is a SEM (scanning electron microscope) photograph showing hillocks that appear in a laminated portion of known transparent conductive thin film (ITO thin film) and metal thin film. The substrate temperature during formation of the ITO thin film is 300.degree. C., and the thickness of the Al film is 50 nm. In the photograph, hillocks are represented by white granular substances that appear in the laminated portion (on the right-hand side of the boundary that extends obliquely in the vertical direction).
The hillocks may be produced for the following reason. There is a large difference in the coefficient of thermal expansion between the substrate and the Al alloy forming the metal thin film, and strains tend to arise inside the Al-alloy film having the larger coefficient of thermal expansion when the substrate and Al-alloy thin film are heated during formation of the ITO film. The hillocks protrude from the surface of the Al-alloy film so as to reduce the strains. To avoid these hillocks, a metallic material for the metal thin film laminated with the transparent conductive thin film has been selected from high-melting-point metals, such as Ta, Ti, Mo, and Cr, which have a relatively low coefficient of thermal expansion. The high-melting-point metals, however, have high resistivity, and the use of such metals undesirably results in an increase in the power consumption in display devices.
In order to reduce power consumed by display devices, it has been reconsidered to use Al-containing alloys having low resistivity for the metal thin film so as to prevent occurrence of hillocks (as proposed in Japanese laid-open Patent Publications No. 5-100248 and No. 8-37186, N. Peacock, Thin Solid Films vol 156 (1988), 173). In the laminate type conductor, however, no attempt has been made to prevent occurrence of hillocks by means of the material of the transparent conductive thin film or method for forming this film, or a new electrode structure.